The present invention generally relates to an automobile steering system and, more particularly, to the steering system having a controllable damper effective to minimize front-wheel shimmy (i.e., low speed shimmy and tramp or high speed shimmy), front-wheel kickback and/or any other undesirable vibration being transmitted to the steering wheel.
The Japanese Laid-open Patent Publication No. 57-57311, published Apr. 6, 1982, discloses an automatically piloted vehicle that runs within the premises of a factory guided by the magnetic field developed in conductors beneath the ground surface. The vehicle disclosed therein is shown as having front road wheels and rear road wheels both being so steerable simultaneously that, when the front road wheels turn in one direction, the rear road wheel can turn in the opposite direction thereby to minimize the turning radius, that is, the radius of circle of turn required for the vehicle to turn. The front and rear road wheel systems have their own linkage systems each connecting the left-hand and right-hand road wheels together, the linkage system for the front road wheels being, however, operatively connected with a reversible servomotor that is controlled by an automatic piloting servocontrol unit in dependence on whether or not the running vehicle has deviated from a predetermined path defined by the conductors beneath the ground surface and whether or not the running vehicle should follow a curved path. The front linkage system for the front road wheels is operatively coupled with the rear linkage system for the rear road wheels by means of a connecting rod so designed as to steer the rear linkage system in the opposite sense to the front linkage system.
For minimizing the vibration of the steering system which obviously includes shimmy and kickback both resulting from external disturbances such as the presence of indents on the ground surface, the above described vehicle employs what is termed as a shimmy damper. The shimmy damper employed therein comprises a double-acting cylinder mounted at one end on the vehicle chassis, a piston member axially slidably housed in the cylinder and dividing the interior of the cylinder into two working chambers on respective sides thereof, and a piston rod connected at one end rigidly with the piston member and at the opposite end with the connecting rod. Ports communicated with the respective working chambers are fluid-connected with an external fluid switching circuit including an electromagnetically operated switching valve. The electromagnetically operated switching valve is adapted to be controlled by the automatic piloting servocontrol unit in such a way that, when and so long as the vehicle runs at a high speed which usually takes place during the straight forward run, the flow of a fluid medium from one working chamber to the other through the switching valve can be blocked or, alternatively, imparted a resistance, to substantially lock the movement of the connecting rod and, hence, the movement of both of the front and rear linkage systems. On the other hand, when and so long as the vehicle runs at a low speed which usually takes place during the turn around a curve, the electromagnetically operated switching valve is opened to permit the free flow of the fluid medium between the working chambers for the ease of steering of the front and rear road wheel systems. In other words, the damping force exerted by the shimmy damper according to the above mentioned prior art publication varies with the running speed of the vehicle and becomes great and small respectively at high speed and low speed run. According to this prior art publication, the use of the vehicle speed as a parameter for controlling the electromagnetically operated switching valve is based on the generalization that the high and low running speeds are utilized during the straight forward run and the rounding along a curved path, respectively, in view of the peculiarity of the vehicle disclosed therein.
Based on the disclosure of the above mentioned prior art publication, it can be contemplated to employ the shimmy damper in a self-propelled manned vehicle such as, for example, a four-wheeled passenger car. In such case, the shimmy damper would be mounted on the automobile so as to extend between a portion of the vehicle body structure and one of the tie rods forming the front linkage system for the front road wheels so that external disturbances acting on the front road wheels during the run of the automobile can be absorbed or substantially suppressed by the shimmy damper thereby to minimize any possible flutter and/or joggling motion of the steering wheel operatively coupled with the tie rods through a steering gear.
Although the contemplated version appears to be effective to minimize the undesirable vibratory motion of the steering wheel, however, even during the steering operation to turn the automobile along a curved path the shimmy damper may exert a relatively great damping force hampering a smooth displacement of the tie rods of the front linkage system. Once this happens, the rounding along the curved path will require the application of a greater steering force to turn the steering wheel than that required when no shimmy damper is employed. This problem appears to be notable when the automobile running at a low or medium speed attempts to round an acute curve at which time the steering wheel has to be turned a great angle about the steering column.